Method of making improved magnesium aluminum gallate phosphors

ABSTRACT

Increased luminence intensity in magnesium aluminum gallate phosphors as well as magnesium gallate phosphors is achieved by utilizing a fluorine source in the raw material mix and by conducting initial processing in a humid, oxidizing atmosphere. Additionally, processing time is drastically reduced.

United States Patent Chenot Apr. 29, 1975 METHOD OF MAKING IMPROVED 3.407.325 lU/l968 Brown 25 /301 MAGNESIUM ALUMINUM GALLATE g:' own e a PHOSPHORS 3.501592 3/l970 Amsler .t t. {75] Inventor: Charles F, Chenot, Tgwanda, Pa 3577.350 4/l97l Amster 252/301 3.595.802 7/1971 Blassc .4 25 /301 1 Asslgfleer GTE Sylvama lnwrporated- 3.649.550 3/1972 Chenot 252/301 Stamford. Conn.

[22] Filed: Mar. 22, 1973 Pr1'mar 1' E.\'aminerJack Cooper Attorney, Agent, or Firm-Norman J. OMalley; 343327 DOnZlld R. Castle; William H. McNeil] Related U.S. Application Data [63] lC(g-rginuznion-in-part of Ser. No, 321651, Feb. 5, ABSTRACT Increased luminence intensity in magnesium alumi- [52] U.S. Cl 252/30l.4 R num gallate phosphors as well as magnesium gallate [51] Int. Cl. C09k 1/04; C09k H68 phosphors is achieved by utilizing a fluorine source in [58] Field of Search 252/30lt4 R the raw material mix and by conducting initial processing in a humid. oxidizing atmosphere. Addition- {561 References Cited ally, processing time is drastically reduced.

UNITED STATES PATENTS 1/1946 Froclich 252/3Ul.fl R

3 Claims, 2 Drawing Figures METHOD OF MAKING IMPROVED MAGNESIUM ALUMINUM GALLATE PHOSPHORS CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of Scr. No. 329.651 filed Feb. 5, i973 and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION This invention relates to fluorescent phosphors and more particularly to magnesium-gallate or magnesium aluminum-gallate phosphors activated by manganese which can be excited by ultraviolet light and still more particularly to an improved method of making improved phosphors of the above type.

Such phosphors are known to emit narrow bands of green light in a manner similar to the emission of pure magnesium gallate activated by manganese as disclosed by Brown in US. Pat. No. 3,407,325. When excited by 253.7 nm ultraviolet radiation these phosphors are particularly applicable to electrostatic, electrophotographic reproduction techniques.

As electrostatic reproduction has improved, new demands have been placed upon the phosphor-lamp combination. These demands have required not only improvements in phosphor-lamp brightness, phosphorlamp maintenance (stability) and phosphor-lamp temperature dependence stability, but changes as well in the actual emission peak position in the spectrum for the luminescent material itself.

It is known that aluminum substitution for gallium in the magnesium gallate type phosphors will cause such an emission shift. As a matter of fact, concerning aluminum substitution in this type of phosphor, the entire spine] crystalline region in the MgOAlgO3-Ga203 system has been studied and reported by Brown (RF. Journal Electrochem. Soc. l 14(3), 245-250 1967). in general, aluminum substitution has been found to shift the peaks of the emission spectra of the phosphor materials activated by mangancse to longer wavelengths (from about 5 lnm to 528nm). However, under short wave ultraviolet excitation accompanying this spectral shift is a decrease in luminescence intensity (measured as total energy by integration of corrected relative spectral energy distribution curves) dropping to about 25 percent for a 50 percent aluminum substitution to virtually 0 percent for a [00 percent aluminum substitution. This significant drop in luminescence intensity with increased aluminum substitution essentially disqualified the high aluminum-containing materials from commercial applications.

Concerning phosphor-lamp dependence stability, the effect of aluminum substitution for gallium in the synthesis of this type of phosphor is reported in British Pat. Nos. l,159,324 and 1.242.983. Therein, temperature dependence stability has been shown to improve substantially for compositions defined by formulae of the type:

MgX(Ga A ylz iil-ai: I nz wherein:

4 2 y a 0.025 and 0 05 a 3 a 0.002 and 0 s a s 1.0

0.002 s b s 0.06

0 s x s 0.96

0 s z s 1.00

0.75 s p s l.l0 and wherein 0.05 S a s 1.0 when 0.90 s 2 L00.

The resulting temperature dependence stability noted for these materials is specified particularly for high-pressure mercury vapor lamp applications where the temperature experienced by the phosphors is often in excess of 300C. The above-cited British patents indicate a similar dramatic decrease in luminescence intensity with substitution of aluminum for gallium as originally shown by Brown; however, emphasis is placed on the fact that with superior temperature dependence stability, the aluminum substituted materials can exhibit superior luminescence intensity at higher temperatures than intensities observed for the more temperature sensitive pure magnesium gallate materials activated by manganese.

Concerning other modifications of the manganese activated magnesium gallate and aluminum-gallate phosphors, the effects on luminescence intensity and emission peak location in the spectrum for germanium and/or silicon substitution for gallium have been reported in British Pat. No. 1,248,373. in particular, luminescence intensity has been shown to be improved substantially for compositions defined by the formula:

xMgO (1 y s zlGa O; yAl O zMO pMnO wherein M0; GeO and/or SiO 0.70 s .r s [.05

Spectral shifts of the emission peaks of these materials were reported in the range from about 501 nm to about 5 l4nm for the above range of compositions.

OBJECTS AND SUMMARY OF THE lNVENTlON It is therefore an object of this invention to enhance phosphor of the above-described types.

It is a further object of the invention to increase the luminescence intensity of aluminum substituted magnesium gallate phosphors.

It is a still further object of this invention to enhance the method of making such phosphors.

These objects are achieved in one aspect of the invention by the provision of a method of manufacturing such phosphors which includes the steps of including in the reaction products of the raw materials a source of fluorine, thoroughly mixing these products and the source of fluorine and heat treating these raw materials in alumina crucibles in a humidified, oxidizing atmosphere for about 2 to 3 hours at a temperature of about l200C- l 300C. After this initial heat treating step the material is cooled and comminuted. Thereafter the material is reheated in silica crucibles in a quite mild reducing atmosphere for about 40 minutes at a temperature of about l200C-l300C. The material is then cooled, comminuted and nylon-screen sieved.

For pure magnesium gallate or low level aluminum substituted magnesium gallate phosphor, the single reduction step tiring of about 40 minutes appears sufficient to achieve maximum luminescence efficiency. With increasing aluminum substitution additional reduction firing steps were found necessary to achieve the maximum efficiency under low pressure mercury vapor excitation. The one or two additional firing steps found most effective were likewise approximately 30 to 40 minutes in length. under the same mild reducing atmosphere, maintained at no less than l300C. and completed by comminution and nylon sieving.

Application of this process greatly reduces preparation time as well as allowing the use of aluminum substitution for gallium with its concommitant spectral shift without adversely effecting luminescence intensity.

DESCRIPTION OF THE DRAWINGS FIG 1 is an elevational view of an aperture type fluorescent lamp which can utilize the phosphors of the invention; and

FIG. 2 is a diagrammatic representation of a gas train and furnace arrangement used in the synthesis of these phosphors.

DESCRlPTlON OF THE PREFERRED EMBODlMENTS For a better understanding of the present invention together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in conjunction with the above-described drawings.

This invention relates particularly to improvements in the preparation of lamp phosphors and to the phosphors themselves and in particular to phosphor materials defined by the generalized formula:

wherein 0.85 sa+b =5 L05 As stated above, a primary purpose of this invention is to provide a new and improved technique for preparing phosphor materials as defined by the immediately preceed'mg formula and wherein luminescence intensities are realized exceeding those normally observed for aluminum substituted materials. In fact, for composi tions defined within the ranges set forth above, manganese-activated phosphor materials can be prepared by this new technique which exhibit substantially greater luminescence intensity when compared to the pure magnesium gallate activated by manganese.

A still further object of this invention is to provide a new technique for the preparation of the subject phosphors wherein synthesis is greatly simplified. Particular emphasis is placed on significantly shortened firing times during heat treatment procedures.

Another particular object is to provide a phosphor material wherein a broad range of emission peaks can be realized. lying in the spectral range from 503nm, for the pure magnesium gallate activated by manganese. to about l 7nm, for compositions near the limit of aluminum substitution defined in the immediately prcceeding formula.

Referring now to the drawings with greater particularity. there is shown in FIG. 1 a lamp l0 which comprises a sealed. hollow glass tube 12 containing a filling of 85 percent argon and l5 percent helium or other suitable gas. On the interior surface of the glass tube 12 there is disposed a coating of phosphor l4. such as the phosphors of the present invention. The phosphor extends around about 3l5 of the circumference of the tube, the remainder l5 being free of phosphor to allow light to emerge therethrough.

At each end of tube l2 there is an electrode comprising an oxide coated tungsten coil. two auxiliary anodes and associated lead-in wires, not shown herein but de tailed in U.S. Pat. No. 2,76! .566. The phosphor can be applied in a known manner.

Luminescent gallate materials as previously known have been synthesized by cumbersome procedures involving either extensive heat treatments, e.g., at temperatures greater than or equal to 1200C for periods of 68 hours, or multiple heat treatments, e.g., at temperatures greater than or equal to 1200C and as high as 1400C for several repeated periods of 2 or more hours per period (accompanies by intermediate comminution and sieving). Such procedures are found necessary to react the raw materials used in gallate phosphor synthesis. Such materials, as finely divided powders, include the highly refractory (and slow to react) B-Ga O MgO or basic MgCO (hydromagnisite), a-Al O (or its slightly more reactive precursor Al- (OH);,) and some manganese compound, e.g., MnCO To obviate the disadvantages of the prior art, an improved method of synthesis is disclosed which utilizes the use of fluorides and an accompanying hydrolysis mechanism. This procedure requires at least partial and as much as total substitution of MgF for the MgO (or basic MgCO used in the prefired raw material blend. in subsequent heat treatments at temperatures greater than or equal to 1 C the raw materials are fired in a humidified, oxidizing atmosphere such as air passed through a sparging unit containing slightly warm water.

Such a heat treatment system is shown in FIG. 2 wherein air is fed into a sparging unit 16 which contains a fritted gas dispenser l8 and warm water 20. The humidified, oxidizing atmosphere generated thereby is fed to furnace 22. The normally sluggish hydrolysis of MgF is enhanced through catalytic action of the finely divided fi-Ga O, and a-Al O during the heat treatment in controlled atmosphere furnace 22.

The overall result is a rapid formation of the desired spine] phase after total consumption of the fluorine through the hydrolysis mechanism according to the rezfitionz l-MgO xMgF- +l yGa O yAl O H O A Mg(Ga -,,Aly)04+2HFT.

Following comminution of the first-step a-l-material, subsequent heat treatments under 0.25-5 percent H in N atmospheres at temperatures greater than or equal to [200C have been found to require no longer than k to A of an hour of firing time, depending on sample size and the level of aluminum substitution. In many cases, second or third step heat treatments carried on for longer than /4 hour were found to be deleterious to the phosphors. These findings regarding the second step heat treatment are very much in accord with second step heat treatment procedures taught by Brown in US. Pat. No. 3.407.325.

The following examples are illustrative of procedures for preparing phosphors according to this invention and are not meant to be limiting.

EXAMPLE 1.

An intimate blend of the following materials is prepared by ball milling:

MgO 0.427 mol (rcl l 3.441 grams/0.2 mol -Continued M F, 0.500 623i 8,0 L000 37.488 MnCO H The blended materials are heat treated for about 2 hours in humid air as described above. The heat treatment is performed in alumina crucibles at temperatures around l200C-l300C. Following this first step heat treatment the cooled material is subjected to comminution and a second step heat treatment. This heat treatment is performed in silica crucibles at temperatures around l200C-l 300C for no longer than 40 minutes, preferably about 30 minutes, in a mildly reducing atmosphere consisting of 0.25-1.00 percent (by volume) H, in 99.75 to 99.00 percent (by volume) N preferably 0.50 percent H, in 99.50 percent N Following this second step heat treatment, the cooled material is subjected to very mild comminution and nylon-screen sieving. The resulting phosphor is a finely divided powder exhibiting an emission peak at about 507nm and a onehalf peak height band width of 32.5nm.

EXAMPLE 2.

An intimate blend is prepared as in Example I:

M30 0.427 mol (reL) 3.442 grams/0.2 mul MgF, 0.500 6.231 B-Gi O, 0.750 28.1 In ALtOHl 0.500 7.970 MnCO, 0.033 0.808

EXAMPLE 3.

An intimate blend is prepared as in Example l:

AnMinlirnute blend is prepared as in Exnrrapze I:

0.427 mol trcl.) 42 grams/0.2 mul MgF, 0.500 6.231 (M0,, 0.550 2am ARCH), 0.900 M346 MnCO 0.033 0.808

The blended materials are three step heat treated in the same manner as that described in Example 2. The resulting phosphor is a finely divided powder exhibiting an emission peak at about 5 l2nm, a one-half peak height band width of 35.0 nm, and a relative emitted radiant energy evaluation of I01 percent when compared with the product of Example 1.

It will be seen from the above that there is herein provided a new and novel process for manufacturing improved gallate phosphors incorporating aluminum to shift the emission peak. Not only is the prior art problem of decreased luminescence intensity eliminated, but in many instances the aluminum substituted material is brighter than the pure gallate. In addition. the processing time is drastically reduced because of the fluorine addition to the raw materials.

While there have been shown and described what are at present considered to be the preferred embodiments of the invention. it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

l. A method of manufacturing magnesium aluminum gallate luminescent material activated by manganese which comprises the steps of: forming a mixture of raw materials which will yield said luminescent material, said materials including a source of magnesium selected from the group consisting of MgO and MgCO a source of gallium as B-Gap and a source of aluminum selected from the group consisting of, a-Al- O and Al(Ol-l) including in said raw materials a source of fluorine as MgF thoroughly mixing said raw materials and said source of fluorine; heat treating said raw materials in alumina crucibles in a humidified, oxidizing atmosphere the water content of said atmosphere being sufficient to effect the hydrolysis of MgF for about 2 hours at a temperature of about l200C 1300C; cooling and comminuting said material; reheating said material at least twice in silica crucibles in a reducing atmosphere for less than 40 minutes each time at a temperature of about l200C l300C; and cooling, comminuting and sieving said material the second and subsequent reheating in said reducing atmosphere effecting an improvement in the luminescence effeciency of said material.

2. The method of claim 1 wherein said luminescent material produced has the general formula gu t z-r c a-"1+0 wherein 3. The method of claim 2 wherein said reducing atmosphere comprises a mixture of H, and N; with said H, being present in an amount of about 0.25 to 1.00

percent by volume.

'0' t! l l 

1. A METHOD OF MANUFACTURING MAGNESIUM ALUMINUM GALLATE LUMINESCENT MATERIAL ACTIVATED BY MANGANESE WHICH COMPRISES THE STEPS OF: FORMING A MXTURE OF RAW MATERIALS WHICH WILL YIELD SAID LUMINESCENT MATERIAL, SAID MATERIALS INCLUDING A SOURCE OF MAGNESIUM SELECTED FROM THE GROUP CONSISTING OF MGO AND MGCO3, A SOURCE OF GALLIUM AS B-GA2O3 AND A SORUCE OF ALUMINUM SELECTED FROM THE GROUP CONSISTING OF, A-AL2O3 AND AL(OH)3; INCLUDING IN SAID RAW MATERIALS A SOURCE OF FLUORINE AS MGF2; THOROUGHLY MIXING SAID RAW MATERIALS AND SAID SOURCE OF FLUORINE; HEAT TREATING SAID RAW MATERIALS IN ALUMINA CRUCIBLES IN A HUMIDIFED, OXIDIZING ATMOSPHERE THE WATER CONTENT OF SAID ATMOSPHERE BEING SUFFICIENT TO EFFECT THE HYDROLYSIS OF MGF2 FOR ABOUT 2HOURS AT A TEMPERATURE OF ABOUT 1200*C-1300*C; COOLING AND COMMINUTING SAID MATERIAL; EHEATING SAID MATERIAL AT LEAST TWICE IN SILICA CRUCIBLES IN A REDUCING ATMOSPHERE FOR LESS THAN 40 MINUTES EACH TIME AT A TEMPERATURE OF ABOUT 1200*C-1300*C; AND COOLING, COMMINUTING AND SIEVING SAID MATERIAL THE SECOND AND SUBSEQUENT REHEATING IN SAID REDUCING ATMOSPHERE EFFECTING AN IMPROVEMENT IN THE LUMINESCENE EFFECIENCY OF SAID MATERIAL.
 2. The method of claim 1 wherein said luminescent material produced has the general formula MgaMnbGa2 c AlcO3 a b wherein 0.85 < or = a*b < or = 1.05 0 < or = c < or = 1.2 0.001 < or = b < or = 0.06.
 3. The method of claim 2 wherein said reducing atmosphere comprises a mixture of H2 and N2 with said H2 being present in an amount of about 0.25 to 1.00 percent by volume. 